A pacemaker is an implantable stimulation device that delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia, a condition in which a patient cannot normally maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate, typically a rate deemed to be appropriate for the patient at rest.
Early advances in pacemakers included the ability to sense a patient's cardiac rhythm. This led to the development of demand pacemakers, so named because they deliver stimulation pulses only as needed by the heart. Demand pacemakers are able to detect spontaneous, hemodynamically effective cardiac contractions that occur within an acceptable time period. This extends the life of the pacemaker's battery as well as avoids competition with the heart's intrinsic rhythm.
The next major advance in pacemakers included the rate-responsive pacemaker which automatically adjusts the patient's heart rate in accordance with metabolic demands. An implanted rate-responsive pacemaker typically operates to maintain a predetermined base rate when a patient is engaged in physical activity at or below a threshold level and gradually increases the paced heart rate in accordance with increases in physical activity until a maximum rate is reached. These pacemakers typically correlate measured physical activity to an appropriate heart rate and define a transition slope between the minimum and maximum heart rate. This transition slope can be telemetrically adjusted to meet patient needs. A common rate-responsive sensor is an activity sensor that transduces mechanical forces associated with physical activity into an electrical signals. Typically, these activity sensors generally contain a piezoelectric transducing element which generates a measurable electrical potential. The pacemaker then analyzes this signal to determine the stimulation rate.
A variety of signal-processing techniques have been used to process the raw activity sensor signals. In one approach, the raw signals are rectified and filtered. Also, the frequency of the highest signal peaks can be monitored. Typically, the end result is a digital signal indicative of the level of sensed activity at a given time. The activity level is then applied to a transfer function which defines the pacing rate (also known as the sensor indicated rate) for each possible activity level. Attention is drawn to U.S. Pat. No. 5,074,302 to Poore et al., entitled “Self-Adjusting Rate-Responsive Pacemaker and Method Thereof”, issued Dec. 24, 1991, which is hereby incorporated by reference in its entirety. This transfer function can be modified telemetrically by the patient's physician. It can also be modified within the pacemaker based upon the stored history of the patient's activity levels to define a new transfer function.
While the rate-responsive pacemaker has very closely mimicked the function of a normal heart during exercise, it was discovered that some patients could not sleep well because either the base rate was too high or they were experiencing short bursts of increased stimulation rate, possibly from sleep movement, that would waken them. This base rate did not accommodate the patient's need for a lower stimulation rate during sleep or sustained rest. A 10–20 beats per minute (bpm) difference can result in difficulty sleeping as well as unnecessarily depleting the pacemaker battery. An example of a rate-responsive pacemaker, which determines when a patient is sleeping and adjusts its base rate accordingly, is set forth in U.S. Pat. No. 5,476,483 to Bornzin et al., entitled “System and Method for Modulating the Base rate During Sleep for a Rate-Responsive Cardiac Pacemaker”, issued Dec. 19, 1995, which is hereby incorporated by reference in its entirety. This reference sets forth methods of modulating the base rate based upon the monitoring of activity variance. By monitoring the variance of an activity signal, it has been shown that one can distinguish between sleep (low variance in the activity signal) and exercises (high variance in the activity signal). This modulated base rate is also known as the circadian base rate. Otherwise, the processor uses the activity transfer function as defined above to determine the stimulation rate.
Unfortunately, there is another group of patients who are not fully assisted with the above stimulation methods. These patients are typically long-term sufferers of diabetes. These individuals have a tendency to gradually lose the function of their autonomic nerves. This is caused by a widespread degeneration of the neurons in the brain and spinal cord due to long term exposure to excess levels of blood sugar. This condition is characterized by a marked decrease in blood pressure upon standing caused by an inability to increase the heart rate and constrict the systemic resistance and capacitance vessels. Healthy individuals, in contrast, can increase their heart rate immediately when they are standing from a prolonged reclined or sitting position. This normal response is called orthostatic compensation. As a result, this patient group has a need for a pacemaker which detects their change in body position from lying or sitting to standing and compensates with an abrupt increase in the pacing rate.
Many different methods have been attempted to determine the physical position of the patient. An example is U.S. Pat. No. 5,354,317 to Alt, entitled “Apparatus and Method for Cardiac Pacing Responsive to Patient Position”, issued Oct. 11, 1994, in which the controller monitors a motion sensor to produce a static output which represents the static position of the patient, i.e. lying down or upright. This static output is used to determine which of the predetermined base rates should be used, i.e. the sleep base rate or the awake base rate. This reference, however, depends upon the detection of a static and stable position. The DC accelerometer, for example, cannot differentiate between the patient lying on the left or right side and standing. It also cannot detect the difference between standing and sitting.
What is needed for this patient population, suffering from a lack of orthostatic compensation, is an abrupt increase in the pacing rate. The '317 reference only teaches to increase the pacing rate upon standing from lying down supine or prone, but does not include standing from sitting or standing from lying on one's side. In addition, the '317 reference depends upon the detection of a static position from a position sensor to determine when to implement the stimulation therapy. The '317 reference also fails to teach the adjustment of the pacing rate to accommodate the patient's sleep cycle.
Accordingly, it is desirable to develop an implantable cardiac stimulation device which maintains the patient's heart rate in relation to the activity level or other metabolic indicator and detects the need for an abrupt increase in the pacing rate upon standing from sitting or lying, thereby mimicking the normal heart's response to orthostatic compensation.